Drilling through rock generally requires the use of some type of fluid to clear cuttings from the bore hole formed by the drill. In some applications, the drilling fluid can be as simple as compressed air. However, when drilling is conducted to tap fossil fuel resources, the drilling fluid used is usually a “drilling mud.” Drilling muds are generally placed in three categories, depending on the major fluid component: water-based, oil-based, and pneumatic. In the natural gas industry, oil-based muds predominate.
Oil-based muds serve several functions during drilling: removing cuttings from the well, suspending the cuttings, controlling formation pressure, sealing permeable formations, stabilizing the wellbore, reducing formation damage, cooling the drill, lubricating the drill, transmitting hydraulic energy to tools and the bit, and reducing corrosion. Oil-based drilling muds typically comprise a hydrocarbon-water emulsion, an emulsifier, and clay. Bentonite is the most widely used clay in drilling muds, although other clays can be used. Other ingredients are often present. Barite, for example, is often used as a weighting agent to increase the outward hydrostatic pressure in the borehole.
Typically, used drilling mud will be recirculated through a drill and borehole at the drill site. The larger cuttings are removed from the mud prior to recirculation. This is generally achieved by running the used mud over a shaker screen. This collects the drill cuttings, which are mixed with drilling mud and groundwater. The waste drilling mud and the cuttings are then subject to disposal, either with or without some form of treatment. In some situations an unused drill mud must be subject to disposal. This can occur for example if a mud is stored for too long, and loses some of its beneficial properties. All such muds, used or unused, are referred to in this disclosure as “waste drilling mud.”
Disposal of waste drilling muds is a major problem in the art. Diesel is commonly used as an oil in drilling muds. Diesel poses environmental hazards, so diesel-based mud must be deposited in special landfills constructed with an impermeable lining. This is expensive, and the possibility remains that the hazardous components of the mud could leak from the landfill, damaging the environment and exposing all parties involved to toxic cleanup liability. Used drilling muds may also contain groundwater with high salt concentrations. Such saline water can also be environmentally harmful if not disposed of properly; its disposal is similarly expensive and can constitute a continuing threat to the environment with attendant legal liabilities. The task of disposal of drilling muds is complicated by the complex, multi-phase nature of the muds, which makes it difficult to isolate the hazardous components to reduce disposal volumes.
Even when the bulk oil fraction of a drilling mud is separated and purified, residual organic compounds often remain tightly associated with solids in the mud (either the clay or drill cuttings), requiring disposal as a hazardous substance. Methods for completely removing hydrocarbons from the solid phase, such as steam distillation, are energy-intensive and inefficient. Solvent-based methods of hydrocarbon separation from the solid phase merely compound the problem by the introduction of hazardous solvent. Combustion of the liquid hydrocarbon in emulsion requires very high operating temperatures and can be a source of air pollution. Combustion of liquid hydrocarbon when mixed with the solid phase is problematic, as it requires the facility be licensed as an incinerator.
If the hydrocarbon fraction is effectively removed, the remaining components of most waste drilling muds (water, clay, and possibly cuttings) are not hazardous, and may be disposed of without special protective measures or reused for muds or other purposes. When waste mud contains saline water, disposal of the aqueous fraction may pose a problem. Although salt concentrations in “saline” groundwaters are low compared to marine waters, they are often sufficiently high to damage soils and bodies of freshwater. Saline water may be disposed of by storage in a lagoon, in which the water slowly evaporates and the salt precipitates. Although this method greatly reduces the volume of the waste material, the concentrated salt evaporite that remains can be highly damaging to soil and groundwater, and requires either alternative disposal or further treatment. Another method of disposal is permanent storage of the saline water in an impermeable landfill. This method is expensive, may result in leaks, and is not available in every location.
Consequently, there is a long-felt need in the art for a method of waste drilling mud disposal that requires no disposal of hydrocarbons and creates no persistent pollution. There is a further long-felt need in the art for a method of waste drilling mud disposal that requires no disposal of saline water. There is a further long-felt need in the art for a method of treatment of waste drilling mud that requires no disposal of hazardous pollutants. There is a further long-felt need in the art for a method of cost-effective diesel recycling from drilling mud.